This invention relates to magnetic sensing apparatus for sensing an external magnetic field and, more particularly, to such apparatus which is formed of magnetoresistive elements and which is substantially insensitive to the particular direction in which the external magnetic field is supplied.
The use of magnetoresistive elements to detect a magnetic field is disclosed in U.S. Pat. Nos. 3,928,836, 4,053,829, and 4,079,360, all assigned to the assignee of the present invention. In these magnetic sensors, ferromagnetic material is deposited in serpentine configuration on an insulating substrate to form two series-connected magnetoresistive elements having respective main current conducting paths which are perpendicular to each other. If a saturating bias magnetic field is supplied to both magnetoresistive elements, a predetermined output signal is produced. In the presence of an external magnetic field, the resultant field through the magnetoresistive elements, that is, the vector sum of the bias and external magnetic fields, will result in a change in the predetermined output signal depending upon the angle formed between the main current conducting paths and the resultant magnetic field.
In the aforementioned patents, a mathematical relation is disclosed between the change in the predetermined output signal and the angular direction in which the external magnetic field is supplied. This expression is as follows: ##EQU1## wherein V is the output voltage produced by the magnetic sensor, V.sub.o is the predetermined output voltage produced as a function of the saturating bias fields, .rho..sub.o is the anisotropic resistance of the magnetoresistive elements and is proportional to the sum of the resistance thereof when a saturating magnetic field is supplied in parallel to the main current conducting path and the resistance thereof when the saturating field is supplied in a direction perpendicular to the main current conducting path (2.rho..sub.o =.rho..sub.11 +.rho..sub..perp.), and .DELTA..rho..sub.o is equal to the difference between the resistance of the magnetoresistive element when the saturating field is supplied in a direction parallel to the main current conducting path and the resistance thereof when the saturating magnetic field is supplied in a direction perpendicular to the main current conducting path (.DELTA..rho..sub.o =.rho..sub.11 -.rho..sub..perp.), and .theta. is the angle between a selected one of the main current conducting paths and the resultant magnetic field. In view of this equation, it is appreciated that, if the bias magnetic fields are of fixed intensity and are supplied in a predetermined direction, the angle .theta. varies in accordance with the angle in which the external magnetic field is supplied to the magnetic sensor.
While the magnetic sensor disclosed in the abovementioned patents operates satisfactorily to detect the direction in which an external magnetic field is supplied, and also functions satisfactorily to detect the proximity of a magnetically permeable member (as disclosed in U.S. Pat. No. 4,021,728), there are some applications in which it is desirable to sense the intensity of the external magnetic field irrespective of the angle at which that external magnetic field is supplied. For example, there may be a need to detect the so-called zero point or zero position of a source of magnetic flux. That is, if the flux source is movable with respect to the magnetic sensor, an output signal should be produced when the flux source admits of a predetermined (e.g. zero) position. In some instances, the orientation of the magnetic sensor and/or flux source may not be uniform, thus resulting in a different angle between the magnetic field generated by the flux source and the magnetic sensor than may be expected. If this angle cannot be predicted or controlled, it is appreciated (from the foregoing equation) that the output signal produced by the magnetic sensor may exhibit a magnitude which is a function of this angle. Hence, because of this factor, the magnetic sensor may produce an output signal which represents that the flux source has reached its zero position even though, in fact, this zero position has not yet been attained. Similarly, when the flux source has, in fact, attained its zero position, the output signal produced by the magnetic sensor may differ from the zero-position output signal.
In view of the foregoing, it has been necessary heretofore to adhere to strict tolerances when assembling a magnetic sensor which is intended to sense the intensity of an external magnetic field. In zero point or zero position sensing applications of the type described above, these assembling tolerances are critical and, moreover, must be maintained throughout the useful life of the apparatus in which the magnetic sensor is used. This has resulted in increased costs of production.